As I said at the end of my post, if you and your girl have an open relationship (which in the originating post you made it sound like you didn't - otherwise you wouldn't have started the thread.... right?) - GO TO TOWN.
Hell, why dont you bring this innocent young damsel up in the bed with you and your older girlfriend and teach her a trick or three?
I dont judge and am about one of THE MOST openminded peeps on here. The types of relationships that I have had have run the gammit.
Honestly, I call bullshit on how you are portraying the reality of the situation as your originating post had one tone and how the tone of your posts since I came down on you a bit has dramatically changed.
I really think you are having a moral delima because one truly exists.
So I will state a third time - if your relationship is as open as you seem to now say - bring the young/inexperienced hottie up in the bed with your gf and teach her a trick or 3.
PROBLEM SOLVED.